1. Field of the Invention
The present invention relates to a beam irradiation apparatus and a beam irradiation method. Moreover, the present invention relates to a method for manufacturing a thin film transistor using the apparatus and the method.
2. Description of Related Art
In recent years, a research has been conducted concerning a thin film transistor having a channel-forming region in a poly-crystalline semiconductor film (hereinafter this thin film transistor is referred to as a poly-crystalline TFT) as a semiconductor element in a display device, an integrated circuit, or the like. With the development of the display device and the integrated circuit, further enhancement of the characteristic of the poly-crystalline TFT has been demanded.
Consequently, as one means to obtain the poly-crystalline TFT, a laser irradiation apparatus including a continuous wave laser and means for scanning a laser beam (also referred to as laser light) is used to crystallize the semiconductor film. (For example, refer to a patent document 1)
Moreover, a galvanometer mirror (refer to a patent document 2) or a polygon mirror (refer to a patent document 3) is often used as the means for scanning the laser light. This is because the galvanometer mirror and the polygon mirror can increase the scanning speed easily. This can reduce the burden on the apparatus.
FIG. 7 shows a structure of a conventional galvanometer mirror. Laser light 72 emitted from a laser light oscillator 71 is reflected by a galvanometer mirror 73 and a spot (irradiated region) 75 is formed on an irradiated object 74. The laser irradiation process is performed in such a way that the spot 75 is scanned on the irradiated object by vibrating the galvanometer mirror 73 at amplitude of vibration 76. The laser light is irradiated to the irradiated object in accordance with the amplitude of vibration of the galvanometer mirror shown in FIG. 7.
In addition, in the case of the polygon mirror, the laser light is irradiated to the irradiated object by rotating a plurality of mirrors provided so as to contact.
Patent Document 1
Japanese Patent Laid-Open No. 2003-86505
Patent Document 2
Japanese Patent Laid-Open No. 2003-86507
Patent Document 3
Japanese Patent Laid-Open No. 2003-45890